Of the many hydroconversion processes known in the art, hydrocracking is becoming increasingly important since it offers product flexibility together with product quality. Because it is also possible to subject rather heavy feedstocks to hydrocracking, it will be clear that much attention has been devoted to the development of hydrocracking catalysts.
In prior disclosures it was not appreciated how to reduce or eliminate the detrimental effects polynuclear aromatics presented to hydroprocessing. Since polynuclear aromatics (PNAs) are somewhat insoluble, their accumulation results in the fouling of process equipment, such as heat exchangers and process lines, and severely impacts upon the hydrocracking catalyst performance. These PNAs also contribute to shortened catalyst life and contribute to higher hydroprocessing temperatures.
In the past, to reduce the amount of PNAs in the feed stream a recycle bleed was utilized. This reduced the PNA concentration in the feed to the hydrocracking process but the practice also resulted in products of value being removed, contributing to process inefficiencies.
Modern hydrocracking catalysts are generally based on zeolitic materials which may have been adapted by techniques like ammonium ion exchange and various forms of calcination in order to improve the performance of the hydrocracking catalysts based on such zeolites.
One of the zeolites which is considered to be a good starting material for the manufacture of hydrocracking catalysts is the well-known synthetic zeolite Y as described in U.S. Pat. No. 3,130,007 issued Apr. 21, 1964. A number of modifications have been reported for this material, one of which is ultrastable Y zeolite as described in U.S. Pat. No. 3,536,605 issued Oct. 27, 1970.
To further enhance the utility of synthetic Y zeolite and depending upon the hydroprocessing problem sought to be solved, additional components have been added by means known in the art. U.S. Pat. No. 3,835,027 issued to Ward et al. on Sep. 10, 1974, which disclosure is incorporated herein by reference, describes a catalyst containing at least one amorphous refractory oxide, a crystalline zeolitic aluminosilicate and a hydrogenation component selected from the Group VI and VIII metals and their sulfides and oxides. Ward et al. teach that the added materials enhance the catalytic and denitrogenation activity of the catalyst.
U.S. Pat. No. 3,897,327 issued to Ward on Jul. 29, 1975, the disclosure of which is incorporated herein by reference, describes a hydrocracking process using a sodium Y zeolite wherein the Y zeolite has a preliminary ammonium ion exchange to replace most of the sodium ion with ammonium ions. This product is then calcined in the presence of at least 0.2 psi of water vapor for a sufficient time to reduce the cell size to a range between 24.40 and 24.64 .ANG.. The patent teaches that the catalyst has increased hydrothermal stability by maintaining crystallinity and surface area after calcination, exposure to water vapor or water vapor at high temperatures.
In addition to various catalyst compositions, preparation techniques have been discovered to also effect catalytic selectivity. U.S. Pat. No. 3,867,277 issued to Ward on Feb. 18, 1975, discloses the use of a Y type zeolite catalyst in a low pressure hydrocracking process. The catalyst described in the patent requires the Y zeolite to be double-exchanged and double-calcined wherein the first calcination step uses a relatively high temperature (950.degree.-1,800.degree. F.) and the second calcination step uses relatively low temperatures (750.degree.-1300.degree. F.) to yield a catalyst that is stable in ammonia environments.
U.S. Pat. No. 3,853,747 issued to Young on Dec. 10, 1974, the disclosure of which is incorporated herein by reference, teaches that hydrocracking activity of the catalyst is greater when the hydrogenating component is incorporated in the zeolite in such a manner as to avoid impregnation into the inner adsorption area of the zeolite crystallites or particles. For example, the mixing may consist of stirring, mulling, grinding, or any conventional procedure for obtaining an intimate mixture of solid material. The dispersion of the Group VIB metal hydrogenation component is achieved by adding it to the zeolite in a finely divided but essentially undissolved form. The patent teaches that in some cases the soluble molybdenum or tungsten compounds added to the zeolite by impregnation tends to destroy the zeolite crystal structure and acidity during the subsequent drying and calcination steps. Young teaches, however, that the particle size should range from 0.5 microns to 5 microns.
U.S. Pat. No. 4,857,171 issued to Hoek et al. on Aug. 15, 1989, the disclosure of which is incorporated herein by reference, teaches a process for converting hydrocarbon oils comprising contacting the oil with a catalyst consisting essentially of a Y zeolite having a unit cell size less than 24.40 .ANG., a silica based amorphous cracking component, a binder and at least one hydrogenation component selected from the group consisting of a Group VI metal, and/or a Group VIII metal and mixtures thereof.
European Patent Application 0,247,678 A2, which is the foreign counterpart of U.S. Pat. No. 4,857,171 issued to Hoek et al. on Aug. 15, 1989, discloses a composition of matter suitable as a catalyst in hydroprocessing comprising a crystalline aluminosilicate zeolite and a binder wherein the crystalline aluminosilicate comprises a Y zeolite having a unit cell size below 24.45 .ANG., a degree of crystallinity which is at least retained at increasing SiO.sub.2 /Al.sub.2 O.sub.3 molar ratios, a water absorption capacity (at 25.degree. C. and a p/p.sub.o of 0.2) of at least 8% by weight of modified zeolite and a pore volume of at least 0.25 ml/g wherein between 10% and 60% of the total pore volume is made up of pores having a diameter of at least 8 nm, an amorphous cracking component, a binder, and at least one hydrogenation component selected from the group consisting of a Group VI metal, and/or a Group VIII metal and mixtures thereof.